1. Field of the Invention
The present invention relates to liquid-cooled brakes, and more particularly, to a fluid-operated liquid-cooled brake assembly used to control or stop a rotatable shaft. 2.
2. Description of the Related Art
Fluid-operated brake devices are used in a wide range of industrial applications. Such devices typically comprise a brake assembly connected to a rotatable shaft, with the shaft extending axially through the center of the brake assembly. The brake assembly also typically includes brake pads or other means utilizing frictional forces to control or stop the rotatable shaft, with the frictional forces controlled through the application of pneumatic pressure. Because the frictional forces of braking can generate large quantities of heat, such brakes often use a flow of liquid (e.g., oil, glycol or water), through the brake assembly as a way to dissipate the heat generated during the braking process.
The oil and natural gas industry is one field in which such fluid-operated liquid-cooled brakes are frequently used. In the oil and natural gas industry, drilling rigs drill wells deep into the earth not only to identify geologic reservoirs but also to create wells that allow the extraction of oil or natural gas from those reservoirs. The drilling rigs use drill strings made of multiple pipe lengths to make the holes in the earth. It is sometimes necessary to remove the drill string from the well bore in a process called tripping pipe, or making a round trip, or simply making a trip. This is done by physically disconnecting, when pulling out of the hole, every other 2 or 3 joints of drill pipe at a time, and then stacking them vertically in a rack in the derrick. During this process, the drill string must be suspended while the pipe lengths are racked. Drilling rigs use liquid-cooled brake assemblies attached to a draw works reel to hold the drill string while making the trip. The brakes are robust, as the drill string is heavy.
Liquid-cooled brake assemblies used in the oil and natural gas industry typically have friction disks that engage a wear plate and convert the potential energy of the heavy drill stem to heat generated through friction. The wear plates become quite hot from the frictional forces, and are cooled with a circulating coolant, most often water, glycol or oil, that passes over a heat transfer insert disposed between the wear plate and a mounting flange or reaction plate. Typical heat transfer inserts are constructed of steel and are formed with many small vertical projections such as nubs or pegs or pins. The nubs permit coolant to circulate therearound in order to transfer heat away from the wear plate.
However, the nubs corrode over time, especially when water is used as the coolant, as the water reacts with the steel. In addition, the nubs suffer from galvanic corrosion due to the use of dissimilar metals between the nubs and the wear plate, mounting flange or reaction plates. The heat transfer inserts, which are made of steel, are disposed adjacent wear plates, which are often made of copper. If water is used, it acts as an electrolyte and thus, a galvanic couple is established. The potential difference between the dissimilar metals is one factor in the accelerated attack on the heat transfer insert. Less heat is conveyed away from the wear plate as the nubs corrode, making the brake less effective as the corrosion progresses.
Corroded heat transfer inserts can be repaired. In usual industry practice, portions of the heat transfer insert are removed via sand-blasting or a similar process, and new steel is welded in place of the removed portions. The new nubs are then machined to be flush with respect to each other and the surrounding surfaces. The repair process is expensive and time consuming, and requires a sophisticated machine shop with large and expensive equipment. The repairs are not suited to be easily performed in the field, where most oil and natural gas drilling rigs are employed. An additional cost to be considered in repairing a liquid-cooled brake is the down-time of a drilling rig while the repair is being performed.
There is thus a need for a liquid-cooled brake assembly that can be easily and quickly repaired when a heat transfer insert is corroded or worn. There is also a need for a brake assembly that can be repaired in the field where it is used. There is a further need for an inexpensive repair for a liquid-cooled brake assembly.